The present invention relates to a method of limiting the charge pressure of an internal combustion engine charged by an exhaust gas turbocharger.
As is conventional, an exhaust gas turbocharger includes a turbine driven by the exhaust gas stream of the internal combustion engine and a compressor driven by the turbine, which pre-compresses fresh air for the internal combustion engine as a function of the charger rotational speed. The dynamic pressure of the exhaust gas stream upstream from the turbine is converted by the turbocharger into charge pressure according to the pressure conversion ratio between turbine and compressor. A higher charge pressure results in a greater air throughput of the engine, increasing its power output. The charge pressure, however, cannot be increased indefinitely to increase the power of the engine. The allowable operating range of the charger compressor is limited by the pumping limit and the maximum allowable rotational speed of the exhaust gas turbocharger.
German Published Patent Application No. 198 08 832 describes a method of controlling the charge pressure in which a control unit performs an actual/setpoint value comparison of the charge pressure with a predefined setpoint value and adjusts the actual value of the charge pressure by controlling a variably adjustable turbine geometry of the charger turbine accordingly. In the method, the setpoint value of the charge pressure is taken from a characteristic map memory as a function of the instantaneous operating state of the engine. The data field for the allowable charge pressure is determined in advance and is stored in the characteristic map memory from which it is read as needed.
In the conventional method, the applied charge pressure setpoint values are determined with a greater safety margin with respect to the pumping limit of the compressor and the rotational speed limit of the exhaust gas turbocharger in order to prevent the limits from being exceeded. An undesirable proximity to the limit value of the exhaust gas turbocharger may result from a plurality of operating conditions of the internal combustion engine, for example, in the event of a changed ambient pressure of the engine, different air temperatures, or when particles accumulate in the air filter after a relatively long operating time. In order to prevent the turbocharger from exceeding its limit values and being damaged due to a clogged air filter in the induction tract of the engine, design measures have been used. Thus, for example, German Published Patent Application No. 196 04 344 describes a bypass to the air filter, which is to be opened as a function of the intake pressure upstream from the air filter.
It is an object of the present invention to provide a method of limiting the setpoint charge pressure, which may allow the internal combustion engine to be operated with the highest possible charge pressure while preventing the turbocharger limits from being exceeded.
The above and other beneficial objects of the present invention are achieved by providing a method as described herein.
According to the present invention, measured values of the temperature and of the pressure upstream and downstream from the compressor of the exhaust gas turbocharger are used for reading the limit value of the controlled variable from the characteristic map memory. These temperature and pressure values are relevant to the proximity of the exhaust gas turbocharger used, to the pumping and rotational speed limits. The maximum possible value of the controlled variable may be predefined with a lower safety margin than previously by consistently detecting these physically relevant variables.
The measured values of the temperature and the measured values of the pressure may be set in relationship to one another and the limit value of the controlled variable is determined from the characteristic map memory using the ratios formed in this manner. The ratios may be formed from the quotients of the respective measured values. In this manner, when the engine is operated at maximum charge pressure close to the charger rotational speed limit or pumping limit of the exhaust gas turbocharger, it may be ensured that the effect of different air temperatures, a possible particle accumulation in the air filter, and other variables having an influence on the operating characteristics and the margin to the charger limits is determined and taken into account.
The pressure and temperature of the intake air may be measured in an air line to the compressor, downstream from an air filter arranged in the clean air line. When measuring at the largest cross-section of the air line, the dynamic portion of the measured pressure value is negligibly small. The pressure drop between the point of measurement and the compressor inlet has barely any effect on the determination of the maximum possible charge pressure and is eliminated as an approximate function of the flow rate in determining the maximum charge pressure values.
In order to read the limit value of the controlled variable from the characteristic map memory, and address value is determined, which may be calculated from the ratios of the measured pressure values and of the measured temperature values and of a measured value of the rotational speed of the internal combustion engine weighted with the volumetric efficiency. When reading the characteristic map memory using the address value, the measured value of the pressure upstream from the compressor, i.e., the intake pressure down stream from the air filter, is taken into account. In an example embodiment of the method according to the present invention, the charge pressure forms the controlled variable of turbine geometry, the maximum allowable charge pressure being represented as a function of the measured pressure value upstream from the compressor and the address value in the case of the limitation according to the present invention. The address value depends on the rotational speed of the compressor and is automatically weighted with the instantaneous volumetric flow rate of the charge air for the internal combustion engine.
An example embodiment of the present invention is explained in detail below with reference to the drawings.